Number 23
The Bells
Kenji hangs a bell on every line Capy is not meant to cross.
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Tripwires
You cannot watch a system continuously, so you set alarms on the specific behaviours you are worried about and let them tell you. The alarms work exactly as designed on the lines you drew, and they are silent everywhere else. A bell can only ring on a line somebody already decided to put a bell on, which makes the set of bells a map of what you were worried about rather than a map of the thing.
In this storyThe bells are not a failure. One rings for a cat in four seconds and Kenji is out of bed and in the garden, which is exactly the job. The trouble comes from the far corner, where the fence quietly gives way and nothing rings, because he never thought about that corner. His answer is to hang more bells, one for every corner he has been wrong about, and he is now up to nine. He does not know how many corners there are. Nobody does.
What the work looks like on safeagi.ca goes into it properly.